1. Field of the Invention
Aspects of the present invention relate to a lithium secondary battery, and more particularly, to a cylinder-type lithium secondary battery having protrusions or depressions formed on a surface of a gasket, which makes contact with a safety vent, so that gas, which is generated inside the battery, and an electrolyte, do not leak, thereby improving the safety of the battery.
2. Description of the Related Art
As generally known in the art, lithium secondary batteries are classified into cylinder-type lithium secondary batteries and square-type lithium secondary batteries according to their appearance. For improved safety, the cylinder-type lithium secondary batteries have a cap assembly adapted to interrupt the current flowing inside the batteries and stop reactions when the internal pressure reaches a critical level and a danger of explosion exists due to overcharging or malfunctioning of the batteries.
Referring to FIG. 1, a cylinder-type lithium secondary battery 100 includes an electrode assembly 200, a cylindrical can 300 for containing the electrode assembly 200 together with an electrolyte, and a cap assembly 400 assembled to the top of the cylindrical can 300 so as to seal it and allow current, which is generated by the electrode assembly 200, to flow to an external device.
The electrode assembly 200 is formed by winding a positive electrode plate 210, which has a positive electrode collector coated with positive electrode active materials on its surface, a negative electrode plate 220, which has a negative electrode collector coated with negative electrode active materials on its surface, and a separator 230, which is positioned between the positive and negative electrode plates 210 and 220 so as to electrically insulate them from each other, into a jelly roll. Although not shown in detail in the drawing, the positive electrode plate 210 includes a positive electrode collector made of metal foil having excellent electric conductivity, such as aluminum (Al) foil, and positive electrode active materials covering both surfaces of the positive electrode collector. Predetermined regions of the positive electrode collector on both ends of the positive electrode plate 210 are not coated with the positive electrode active materials and define positive electrode uncoated regions. A positive electrode tab 215 is bonded to an end of the positive electrode uncoated regions. The positive electrode tab 215 is generally made of aluminum (Al) and protrudes a predetermined distance from the top of the electrode assembly 200.
The negative electrode plate 220 includes a negative electrode collector made of metal having electric conductivity, such as copper (Cu) or nickel (Ni) foil, and negative electrode active materials covering both surfaces of the negative electrode collector. Predetermined regions of the negative electrode collector on both ends of the negative electrode plate 220 are not coated with the negative electrode active materials and define negative electrode uncoated regions. A negative electrode tab 225 is bonded to an end of the negative electrode uncoated regions. The negative electrode tab 225 is generally made of nickel (Ni) and protrudes a predetermined distance from the bottom of the electrode assembly 200. The electrode assembly 200 may further include insulation plates 241 and 245 positioned on its top and bottom, respectively, so that contact with the electrode assembly 200 or the cylindrical can 300 is avoided.
The cylindrical can 300 includes a cylindrical lateral plate 310 having a predetermined diameter so as to define a space for receiving the cylindrical electrode assembly 200 and a lower plate 320 for sealing the bottom of the cylindrical lateral plate 310. The cylindrical lateral plate 310 has an open top so that the electrode assembly 200 can be inserted through it. The negative electrode tab 225 of the electrode assembly 200 is bonded to the center of the lower plate 320 of the cylindrical can 300, which then acts as a negative electrode. The cylindrical can 300 is generally made of aluminum (Al), iron (Fe), or an alloy thereof. The cylindrical can 300 has a clipping portion 330 formed on its upper end so as to bend inwards, in order to compress the top of the cap assembly 400, which is coupled to the upper opening of the can 300. A part of the cylindrical can 300, which is spaced from the clipping portion 330 in the downward direction by as much as the thickness of the cap assembly 400, is indented so as to form a beading portion 340, which compresses the bottom of the cap assembly 400.
The cap assembly 400 includes a safety vent 410, a current interruptor 420, a secondary protective device 480, and a cap-up 490. The safety vent 410 has the shape of a plate, the center of which protrudes downwards, and is positioned on the bottom of the cap assembly 400. The protrusion at the center of the safety vent 410 is deformed upwards by pressure occurring inside the secondary battery. One of the positive and negative electrode tabs 215 and 225, which are drawn from the positive and negative electrode plates 210 and 220, respectively, is welded to a predetermined portion of the lower surface of the safety vent 410 so that the safety vent 410 is electrically connected to a corresponding one of the positive and negative electrode plates 210 and 220 of the electrode assembly 200. The other of the positive and negative electrode plates 210 and 220 is electrically connected to the can 300 via a tab (not shown) or directly connected thereto.
In the case of a conventional cap assembly, it is customary to insert a gasket into a can, followed by a safety vent, a current interruptor, a secondary protective device, and a cap-up and subject the can to crimping so that gas, which is generated inside the battery, and an electrolyte, do not leak. However, such a conventional structure has a problem in that the attachment condition between the safety vent and the gasket may degrade, and when the battery's internal pressure rises, gas may leak from inside the battery due to the poor sealing condition. In addition, when the electrolyte penetrates into a narrow gap between the cap assembly and the gasket or between the gasket and the cap, due to external impact, for example, the electrolyte may leak out of the battery due to a capillary phenomenon or cohesion force, for example.